On Thu, 19 Dec 2002 03:35:09 GMT, jones144
>On 18 Dec 2002 12:18:22 -0800, rcochran@lanset.com (Richard Cochran)
>wrote:
>>jones144
news:
>>> On Wed, 18 Dec 2002 00:34:29 GMT, "Tony Spadaro"
>>>
>>> Which brings me to a question thats been on my mind for some time. I
>>> have a D100. When I put my Nikkor105/2.8 Micro on it, using it in
>>> aperture priority, with the lens set at infinity and aperture set at
>>> 2.8, the camera indicates 2.8 but as I begin focusing closer than
>>> infinity the aperture reading changes, until I get to close focus
>>> where it shows f5 even though I haven't changed the aperture dial.
>>> Focusing to infinity brings the aperture back to f2.8.
>>> This does the same thing on my F100.
>>> I get 2.8 only at infinity, as soon as focusing gets to about 7 feet
>>> aperture changes to 3.0 and progressively gets smaller up to f5 at
>>> closest focus, about 1 foot.
>>> I know the variable aperture zooms will do this, but a prime lens?
>>> What gives? Is this due to the distance required by the front element
>>> travel from infinity to close focus? My only other prime is a 50/1.8
>>> and that does not change aperture when focusing.
>>>
>>> Do prime lenses other than micro/macro lenses do this?
>>Yes and no. They all change effective aperture when focusing,
>>but most don't display it in the viewfinder. At normal
>>(non-macro) focusing distances, the change is normally
>>small enough that it's not worth bothering with.
>>
>>Remember the effective f stop number is the lens-to-film distance
>>divided by the aperture diameter. When focused at infinity,
>>the lens-to-film distance is the same as the focal length printed
>>on the lens barrel. But when focused closer, the lens is
>>moved further away from the film (I'm assuming a lens with no
>>floating elements, though some of the following will be
>>more or less true even for some lenses with floating elements).
>>
>>When you move the lens further from the film, you increase the
>>f number. The formula is:
>> Effective f-number = Marked f-number * (1 + Magnification)
>>
>>Where the Magnification is defined as the image size (on film)
>>divided by the subject size. Your Nikkor is just doing this
>>computation for you, and displaying the effective f-number in
>>the viewfinder. You can see that the compensation only becomes
>>significant when you're focused quite close.
>>
>>Do a web search on "bellows compensation factor" for more details.
>>
>>--Rich
>Thanks rich.
>After reading David Littlewood's and your posts, and reading up on the
>bellows factor, I think I see how this happens.
>
>The f5 at close focus is fine with me as I use f11 or smaller aperture
>when using the macro function, and usually manual focus. I just felt a
>bit cheated at first when I discovered this.
>Thanks to both of you for responding.
This question comes up often, and the others answered
it well. BTW, there is only one macro lens I know of
that remains constant-aperture with focus (but not with
zooming - which I think is an opportunity lost...! ;-),
and that is the 70-180 f4.5-5.6 Micro-Nikkor. While it
appears slow compared with most macro lenses, at
closest-focus, it is roughly the same speed as others.
Too bad it was not constant-aperture with zooming, also,
even if it would then be f5.6 throughout, since metering
could then be done with a hand-held meter (often easier
to use than TTL metering [but for the "bellows factor]
for macro, since the majority of the image may be
occupied by green, white, or a strong color like red,
orange, or yellow - these can result in metering errors
unless compensated for...).
BTW, there is a review of the 70-180mm Nikkor, listed
at: www.David-Ruether-Photography.com/articles.html.